(1) Field of the Invention
The present invention relates to radar apparatuses using spread spectrum scheme, and particularly to a high performance spread spectrum radar apparatus which is inexpensive and has a wide detectable range.
(2) Description of the Related Art
In recent years, technology for radar apparatuses equipped on automobile (hereinafter referred to as in-vehicle radar apparatus) has been actively developed. The in-vehicle radar apparatus aims at improvement in safety such as crash avoidance and improvement in driving convenience including support when the user starts a car backwards, and is used for detecting preceding vehicle and obstacles in the back, for example. For these purposes, it is necessary to suppress the influence of unnecessary waves such as interference caused by electromagnetic waves from an in-vehicle radar apparatus of the same type equipped in another vehicle.
In order to solve the problem, a radar apparatus using the spread spectrum scheme (hereinafter referred to as a spread spectrum radar apparatus) has been proposed (See Patent Reference 1: Japanese Unexamined Patent Application Publication No. 7-12930).
In the spread spectrum radar apparatus, transmission waves are spread using a pseudo noise code (hereinafter referred to as PN code), and the receiver despreads the transmission waves using the same PN code used for modulating the transmission waves. This restrains waves modulated using different codes and waves radiated from a radar apparatus using other schemes which does not involve code modulation in the receiver, and thus suppresses interference. Furthermore, since the transmission waves are frequency spread using the PN code, electricity per frequency unit may be reduced, and the effect to other wireless system may also be lowered. Moreover, the relationship between distance resolution and the maximum detectable range may be freely set by adjusting the chip rate of PN code and the cycle of code. Furthermore, the peak electricity of the spread spectrum radar apparatus is not large compared to a radar apparatus using the pulse scheme since the waves can be sequentially transmitted. Thus, the spread spectrum radar apparatus is advantageous in reduction of the amount of electricity necessary for a high-frequency circuit and providing inexpensive radar apparatuses.
A radar apparatus having a high-frequency circuit simplified by using a common local oscillator on the transmission side and the reception side, and in which the polarity of PN code is reversed at an appropriate time has been proposed as a specific structure of the spread spectrum radar apparatus (see Patent Reference 2: Japanese Unexamined Patent Application Publication No. 10-54874). The reception unit of the radar apparatus in Patent Reference 2 is configured to quadrature-demodulate the received signal directly using transmission carrier wave. Reversing PN codes used for despreading at an appropriate interval reverses polarity of the quadrature-demodulated output at the appropriate interval, makes the quadrature-demodulate output as alternating signal and the influence of direct-current offset is eliminated. As described above, the radar apparatus is a good radar apparatus which is not subject to the direct current offset caused by the variation in characteristic of semiconductor devices and variation of external temperature.
Operations of the radar apparatus when reversing the PN code is described specifically using an M-sequence code as the PN code.
The M-sequence code is a PN code which includes “0” and “1” as logical values, and the number of “1” included in one cycle is always one more than the number of “0”. In addition, as shown in FIG. 1 the M-sequence code has good correlation characteristics. FIG. 1 shows the correlation characteristics of an example M-sequence code which has 7 bits per cycle. As shown in FIG. 1, when calculating correlation of one M-sequence code and the M-sequence code shifted to a few bits, the correlation value is the peak value, or 7 when the shift amount is 0, and the correlation value is −1 in any other case. Note that the correlation value is calculated by subtracting the number of difference from the number of matching two. The correlation value is calculated by converting “0” in the M-sequence code to “−1”, and calculating the product sum.
As described above, the correlation value takes a large value only when an M-sequence code and another M-sequence code match, and the correlation value is extremely small in other cases. In the conventional technology, distance to an object can be detected using the correlation characteristics.
However, the conventional technology has a problem that the sensitivity of the radar apparatus falls due to transient impulse noise generated in the demodulated output, even in the non-correlated signal which is essentially suppressed with correlation characteristics of the PN code, continuity of the code is lost in the instant that the PN code is reversed. The problem is hereafter described in detail.
The technology described in Patent Reference 2 describes reversal of PN code for suppressing the influence of direct current offset. As shown in FIG. 2, there could be irregularity in the number of “0” and “1” when the cycle for obtaining the correlation characteristics includes the instant when the M-sequence code is reversed. In this case, the distinct correlation as shown in FIG. 1 is not obtained. As a result, when the spread signal spread using the PN code is despread using the reversed PN code, an impulse noise as shown in FIG. 3 is generated in the signal after despread or quadrature-demodulation. The impulse noise shown in FIG. 3 is generated in the instant when the PN code is reversed.
The waveform shown in FIG. 3 shows the output waveform, for example, when the PN code passes through a low-pass filter of a fully-wide bandwidth when the cycle of PN code is 2047 bit, the chip rate is 2500 Mcps and the frequency for reversing the PN code is 50 kHz. In this example, components approximately over 1.2 MHz are fully suppressed. Amplitude component determined by the correlation characteristics of the PN code is generated in the part where the impulse noise is eliminated. As shown in FIG. 3, the output waveform is reversed according to reversal and non-reversal of PN code. Furthermore, when there is an object reflecting the spread signal, a reflected wave of amplitude according to the reflection intensity of the object and the distance from the object to the radar apparatus.
For example, as shown in FIG. 3, when the first reflected wave having larger amplitude than the amplitude of the impulse noise, it is possible to detect the reflected wave in the radar apparatus. Meanwhile, in the case of the second reflected wave having smaller amplitude than the amplitude of the impulse noise, it is difficult to detect the reflected wave.
Thus, the influence of the impulse noise causes a problem that the sensitivity of the radar apparatus falls, for example, reduction in the maximum detectable range.